Oil Separation in a Cooling Circuit

ABSTRACT

The invention concerns a method and a cooling system for oil separation in a cooling circuit, including compression means ( 4 ) that supply coolant under a first high pressure through means for oil separation ( 10 ) and further on through condensing means ( 12 ), from where coolant is applied restriction means ( 16 ), from where coolant is conducted to flooded evaporation means ( 18 ), from where the compression means ( 4 ) suck gaseous coolant back, where remaining oil in the flooded evaporation means is returned to the cooling circuit through an oil removal circuit ( 26 ). It is the purpose of the invention to achieve an efficient oil separation from a flooded evaporator ( 18 ). This may be attained by the cooling system according to the invention, in that the oil removal circuit ( 26 ) includes a pump ( 24 ), the suction side of which being connected to the flooded evaporator ( 18 ), where the pressure outlet of the pump is connected to a pressure connection ( 9 ) between the pressure outlet of the compressor ( 4 ) and the oil separator ( 10 ).

The invention concerns a method for oil separation in a cooling circuit,including compression means that supply coolant under a first highpressure through means for oil separation and further on throughcondensing means, from where coolant, mainly in liquid state, is appliedrestriction means, from where coolant is conducted to floodedevaporation means under a second lower pressure, from where thecompression means suck gaseous coolant back, where remaining oil in theflooded evaporation means is returned to the cooling circuit through anoil removal circuit.

Moreover, the invention concerns a cooling system containing at leastone compressor having a pressure outlet, which is connected to an oilseparator, where the oil separator is further connected to at least onecondenser, from where the coolant is conducted through a coolant line toat least one restriction element communicating with at least one floodedevaporator, from where primarily gaseous coolant flows to the suctionside of the compressor, where an oil removal circuit from the coolantcircuit is provided in connection with the flooded evaporator.

In cooling systems, coolant and oil used for lubricating the compressorcome into contact with each other. Thus a certain amount of oil isconducted on to the cooling system. In well-known systems with so-calledflooded evaporators, oil will be concentrated and accumulated in theevaporator. Coolant vapours sucked from the flooded evaporator to thecompressor contain practically no oil. Therefore, oil from the floodedevaporator is to be returned to the compressor.

Depending on properties and quantitative ratio between oil and coolant,the oil may remain in a separate state or be mixed partly or wholly withthe coolant. In the first case, the oil is immiscible; in the secondcase, the oil is wholly or partly miscible. Irrespectively of themiscibility, oil is normally unwanted in the evaporator as the oilreduces the heat transmission coefficient and thus the efficiency of theevaporator, and the oil disappearing from the compressor is to besubstituted by refilling. The oil is therefore to be removed fromflooded evaporators. If the oil is not miscible, and if the oil isheavier than the coolant, the oil is collected on the bottom of theevaporator due to the force of gravity. From here, the oil is removedmanually by draining off. Due to the manual operation, this method israrely used today. Alternatively, the draining can be automatised inthat the oil by means of the force of gravity is collected in a smallcontainer connected in proximity of the evaporator, from where it ispressed to the suction side or the crank housing of the compressor,preferably by means of hot gas. The system can be activated when thecollected amount of oil exceeds a determined value.

The disadvantage of the last-mentioned solution is that compressors, andparticularly piston compressors, cannot stand up to liquid coolant oroil at the suction side of the compressor due to the risk of hammering.Therefore, strict requirements are made for indicating when only oil ispresent in the collecting container. It is also a problem that the onlyforce driving the oil to the container is the force of gravity, whichmeans that the driving force may be insufficient at low temperatureswhen the oil viscosity is high.

For systems with coolant miscible with oil, there is described asolution in “ASHRAE HANDBOOK System Practices for HalocarbonRefrigerants”, 2.29 (FIG. 1.), published 2002. By this solution, thesuction line of the compressor is to be passed under the liquid level ofthe evaporator in order to ensure sufficient driving height for liquidtransport, implying a difficult disposition of the suction line.

Another solution for removing miscible oil is found in “PohlmannTaschenbuch der Kältetechnik”, 16. Auflage punkt 7.9.4.4. Bild 7-138,published 1978. In this case, a small dry evaporator is used, alsocalled an oil heater for coolant evaporation, which is connected to thecoolant circulation pump and in parallel with the flooded evaporator. Adry evaporator is capable of transporting the oil due to a high coolantgas speed from the dry evaporator/oil heater. The capacity of the smalldry evaporator/oil heater is selected so that the maximum oil content inthe flooded evaporator is not exceeded. The oil heater is mounted belowliquid level in the flooded evaporator. If the oil heater is filled upwith clean oil in case by a interruption of operation, the system cannotstart up again by itself as the clean oil does not evaporate andtherefore has to be removed manually from the oil heater. Furthermore,it is rare that flooded evaporators have a coolant recirculation pump.

In connection with miscible oils and with regard to both solutions thereis, however, still the drawback that hammering may occur in thecompressor.

A common problem to all solutions for both miscible and immiscible oilsis that oil is transported to the suction side of the compressor. Sinceflooded evaporators form saturated vapours practically withoutsuperheating, there is only very little opportunity of adding extraliquid coolant for oil removed from the evaporator. As the added liquidcannot evaporate in saturated vapours, it remains in liquid state andmay cause hammering which may destroy the compressor.

It is the purpose of the invention to achieve an efficient oilseparation and returning of oil from a flooded evaporator. A furtherpurpose may be to control the oil level in a compressor.

This may be attained by the cooling system according to the invention,in that the oil removal circuit includes at least one pump, the suctionside of which being connected to the flooded evaporator, where thepressure outlet of the pump is connected to a pressure connectionbetween the pressure outlet of the compressor and the oil separator.

By the invention, the oil accumulated in the evaporator is not suppliedto the suction side of the compressor with the resulting risk ofhammering. The pump delivers a mixture of cold liquid coolant and oil tothe pressure side of the compressor in the circuit, where the coolant ispresent in superheated gas state, and therefore the coolant in gaseousstate may absorb and evaporate the liquid coolant while the oil remainsin liquid state. When the mixture of superheated coolant and oilsubsequently passes through the oil separator, the larger part of theoil is removed. At the same time it happens that the efficiency of theoil separator rises, as the coolant gas mixture has lower temperaturedue to the heat energy used for evaporating the liquid supplied to thecompressed hot coolant gas.

The oil removal circuit may advantageously be designed so that thepressure side of the pump communicates with pressure control valves fromwhere excess coolant is conducted to the flooded evaporator. Hereby maybe achieved that the oil removal circuit can remove a substantial amountof liquid coolant from the bottom of the evaporator, withoutsubstantially reducing the efficiency of the cooling system.

The cooling system may contain a number of compressors having coolantcommunication through oil separators and further on through at least onecondenser to a number of expansion elements, from where coolant may beconducted to a number of independent evaporators where floodedevaporators advantageously may be connected to the oil removal circuit,where an outlet connections from the oil removal circuit may form aconnection to a central oil distribution circuit, from where oil isconducted to the active compressors, depending on the actual oil levelin each individual compressor. Hereby may be achieved an efficientautomatic regulation of the oil supply to a number of compressors. Theneed for checking and refilling oil on individual compressors is thusreduced.

The mixture of oil and liquid and/or gaseous coolant may advantageouslyflow through a preheater. Before the mixing, the pressure connectionbetween the pressure outlet of the compressor and the oil separator isprovided. Hereby may be achieved that the mixture of oil, liquid andgaseous coolant returning from oil removal circuit is preheated, wherebya part of the fluid coolant is provided in gaseous state.

Advantageously, the preheater may be contained in an oil separator.Hereby may be achieved a cooling of the oil collected in the oilseparator.

The method and circuit according to the invention may be used for bothmiscible and immiscible oil, and the pumps used in the oil removalcircuit may transport both of these oils. Furthermore, it is possible tosuck an oil/coolant mixture from the evaporator, where the amount ofcoolant is substantially greater than the amount of oil, and thus acoolant circulation arises in the evaporator itself as replacement tothe sucked off coolant. This extra internal coolant flow also entrainsoil located far from the suction branch. In that way, the oil removalaction is increased in case of immiscible oil, compared with othertechnical solutions where the oil flow toward the oil removal circuit isonly based on the force of gravity. This has particularly significancewith low temperatures where the viscosity of the oil can be high and theoil flow thus be impeded.

By using screw compressors in the cooling circuit, a considerably amountof oil may also be circulated for compressor lubrication and for coolingpressurised gases. The oil itself may therefore also be cooled. In manycases, this is effected by injection of coolant. If the amount of thepumped coolant is selected so great that, besides removing the oil, theneed for cooling pressurised gases is also covered, the extra equipmentfor cooling pressurised gases may be done without. In that way, acombined oil removal/oil cooling system may be provided.

As pump in the oil removal circuit, one may use the common mechanicalpumps, e.g. gear wheel pumps, but other alternatives are possible, e.g.ejector pumps, MHD-pumps, gas-driven pumps etc.

In the following, the invention is explained from

FIG. 1 showing a possible embodiment of the invention, and from

FIG. 2 showing an alternative embodiment of the invention.

On FIG. 1 is shown a cooling system 2 containing at least one compressor4 having a coolant inlet 6 and a coolant outlet 8. The coolant outlet ofthe compressor 4 communicates with an oil separator 10, where the oilseparator 10 conducts oil back to the compressor 4 over a connection 20.From the oil separator 10 the coolant is conducted through a condenserunit 12, where the coolant changes its state from mainly gaseous tomainly liquid state. With a coolant connection 14, coolant is conductedto a restriction element 16, whereafter coolant is conducted to aflooded evaporator 18 where it evaporates. From the evaporator 18, thecoolant is conducted to the compressor 4 through coolant inlet 6. At thebottom of the flooded evaporator 18 there is provided an oil drain inthe shape of a connection 22 communicating with a pump 24. A connection26 is shown from the pump 24, connected to a restriction element 28 fromwhere a connection 30 forms connection further on to a point 9 in theconnection between the pressure outlet 8 of the compressor 4 and the oilseparator 10. Possibly redundant coolant is conducted from theconnection 26 back to evaporator 18 through restriction element 32 andconnection 34.

The exemplified cooling system 2 can be provided with a miscible or animmiscible oil.

An oil removal circuit 20, 22, 26, 30 is arranged for transporting oilmixed with coolant away from the flooded evaporator 18. The connectingline 22 may advantageously be situated below the liquid level in theevaporator 18 and form a connection to a point 9 in the line or theflowpath between the pressure outlet 8 of the compressor and the inletof the oil separator 10. The liquid transport in the oil removal circuitis effected by means of a gear wheel pump 24 that imparts a suitablyhigh pressure to the oil/coolant mixture, the pressure being furtherregulated by a orifice plate/control valve 28 for controlling thecirculated amount of liquid before discharge at the point 9.

In order to separate the possible excess coolant, this is returned tothe evaporator through a bypass line 34, and through a control valve 32.

By the method according to the invention, the pump 24 sucks a limitedamount of coolant/oil mixture from the flooded evaporator 18 and pumpsit into the pressure line of the compressor at the point 9, wherebyhammering in the compressor 4 is avoided, as the superheated gas in thepressure line can absorb liquid which is evaporated due to the hightemperature. At the same time, the superheating of the coolant isreduced, enhancing the efficiency of the oil separator. In the existingoil separator 10, the oil is separated and led to the compressor 4.

The system may be automatically restarted, as the pump may start withclean oil.

FIG. 2 shows an alternative embodiment of the invention, where thecompressor 4, as shown on FIG. 1, has a pressure outlet 8 connected toan oil separator 110. From the oil separator 110, coolant is conductedon towards a not shown condenser via a connection 14. A pump 24 is shownwith a connection to a not shown flooded evaporator. From the pump 24,there is a connection to restriction elements 28 and 32. From therestriction element 28 there is shown a connection 130 containing a heatexchanger 42 disposed inside the oil separator 110. From the heatexchanger, the connection 130 continues to a connecting point 9 in thepressurised gas connection 8. The oil separator 110 contains an oilseparator element 40 disposed uppermost in the oil separator, and belowthere is shown an oil level 44 where oil is conducted to the compressor4 via the connection 20.

By the embodiment shown on FIG. 2 is achieved that the mixture of oil,liquid and gaseous coolant contained in the connection 130 is preheated,whereby a part of the liquid coolant is provided in gaseous state. Atthe same time, cooling of the oil collected in the oil separator may beachieved.

1. A method for oil separation in a cooling circuit (2), including compression means (4) that deliver coolant under a first high pressure through means for oil separation (10) and further on through condensing means (12), from where coolant in liquid state is applied restriction means (16), from where coolant under a second lower pressure is conducted to at least one flooded evaporation means (18), from where coolant, which is mainly in gaseous state, is conducted to the suction side of the compression means (4), where remaining oil in the flooded evaporation means (18) is returned to the cooling circuit (2) through an oil removal circuit (22, 24, 26, 28, 30), characterised in that the oil removal circuit (22, 24, 26, 28, 30) includes means (24) for pumping oil from the flooded evaporation means (18) and to a point (9) in a pressure connection forming a coolant connection between the pressure outlet of the compression means (4) and at least one oil separation means (10).
 2. A cooling system (2) containing at least one compressor (4) having a pressure outlet (6), which is connected to an oil separator (10), where the oil separator (10) is further connected to at least one condenser (12), from where the coolant is conducted through a coolant line (14) to at least one restriction element (16) communicating with at least one flooded evaporator (18), from where primarily gaseous coolant flows to the suction side (6) of the compressor, where an oil removal circuit (22, 24, 26, 28, 30) connected to the coolant circuit (2) is provided in connection with the flooded evaporator (18), characterised in that the oil removal circuit (22, 24, 26, 28, 30) includes a pump (24), the suction side (22) of which being connected to the flooded evaporator (18), where the pressure outlet of the pump is connected to a point (9) in a pressure connection between the pressure outlet of the compressor (8) and the oil separator (10).
 3. Cooling system according to claim 1, characterised in that the outlet (8) of the pump (24) communicates with pressure control valves (28-32), from where excess coolant is conducted to the flooded evaporator (18) via pressure control valves (32).
 4. Coolant (2) according to claim 1, characterised in that the cooling system (2) contains a number of compressors (4) having coolant communication through oil separators (10) and further on through at least one condenser (12) to a number of expansion elements (16), from where coolant is conducted to a number of independent evaporators (18) where flooded evaporators (18) are connected to the oil removal circuit (22, 24, 26), where an outlet connection (26) from the oil removal circuit forms a connection to a central oil distribution circuit, where oil is conducted to the active compressors depending on the actual oil level in each individual compressor.
 5. Cooling system according to claim 1, characterised in that the mixture of oil and fluid and/or gaseous coolant flows through a preheater (42) before the mixture is supplied to the pressure connection (8) between the pressure outlet of the compressor and the oil separator (110).
 6. Cooling system according to claim 5, characterised in that the preheater (42) is contained in the oil separator (110). 